1. Field of the Invention
The present invention relates to a receiving technology and, in particular, to a receiving apparatus for receiving signals through an adaptive array antenna.
2. Description of the Related Art
An adaptive-array antenna technique is a technique in which a plurality of antennas are arranged in array and the directivity is dynamically varied according to a radio wave environment. The function of adaptive array antennas is divided into an adaptive beamforming and an adaptive null steering. In the adaptive beamforming, a main beam is directed toward the arrival direction of signals to be received. In the adaptive null steering, the null in a directional pattern is formed in a direction of interference wave (See Reference (1) in the following Related Art List, for instance).
In the FM radio broadcasting, radio waves using a frequency modulation (hereinafter referred to as “FM”) method are used to broadcast audio signals, such as verbal exchange (conversation) and music, to the general public. The FM conveys information through a change in frequency of carrier wave. Specifically, the frequency of carrier wave is lowered or raised by a signal voltage. As a result, the signal is expressed by the density degree of carrier wave. The maximum value and the minimum value of frequency are defined as a minimum frequency transition. For example, the maximum frequency transition is defined as ±75 kHz in the Radio Act. In the Radio Act, the occupied bandwidth up to 200 kHz (±100 kHz) is assigned to each FM broadcast station, based on the constraint of the maximum frequency transition of 75 kHz and 15 kHz for the signal wave frequency.
The use of adaptive array antennas is effective in enhancing the receiving characteristics of such FM radio broadcasting. If, however, the received power of signal coming from a station that one does not wish to receive the signal (hereinafter such a station will be referred to as “interfering station”) is larger than that of signal coming from a station that one wishes to receive the signal (hereinafter such a station will be referred to as “desired station”), the main beam are sometimes directed toward the direction of an interfering station in the adaptive array antenna. To reduce the impact of the interfering stations, band limiting can be performed in such a manner that the signal only coming from the desired station is extracted. However, such band limiting causes the audio quality to deteriorate. Further, the radio wave environment varies if the receiving apparatus changes its location. Thus, a weight vector of the adaptive array antenna also varies. Thus, the receiving condition of signals will be more stabilized if the weight vector thereof can be estimated and then controlled.
Similar to the transition occurring from analog television broadcasting to digital television broadcasting, a transition from analog radio broadcasting to digital radio broadcasting is under consideration. One way to realize the transition, digital radio signals are broadcast independently of analog FM radio signals, in a similar manner to the television broadcasting. Another way to realize the transition, signal is broadcast in such a manner that the digital radio signal is appended to the analog FM signal. In the latter case, signals in which analog FM signals and digital radio signals are combined together (hereinafter such combined signal will be referred to as “hybrid signal”) are broadcast, so that this may be called hybrid broadcasting. In the hybrid broadcasting, digital data with high sound quality are reproduced if the receiving apparatus can receive digital radio signals. The conventional analog data are reproduced if the receiving apparatus cannot receive digital radio signals. Accordingly, no burden of urgent equipment investment is placed on broadcasters or general users in the hybrid broadcasting. To improve the receiving characteristics of such hybrid broadcasting, the use of adaptive array antenna is effective. Note that different types of signals are contained in the hybrid broadcasting. As a result, adaptive array processing must be carried out even when the different types of signals are contained therein.